1. Field of the Invention
The present invention relates to a method for screening the magnetic field generated by an electrical power transmission line. The present invention also relates to a magnetically screened electrical power transmission line.
2. Description of the Related Art
Generally, a high-power electrical power transmission line is designed to withstand voltages of the order of hundreds of kV (typically 400 kV) and currents of the order of thousands of amperes (typically 500-2000 A). The electrical power carried in these lines can reach values of the order of thousands of MVA, typically 1000 MVA. Normally, the current carried is an alternating current at low frequency, in other words generally below 400 Hz, and typically at 50-60 Hz. In general, these lines are used for transferring power from an electrical power station to a city, over distances of the order of tens of km (normally 10-100 km).
In a typical configuration, a three-phase line comprises three cables buried in a trench with a depth of 1-1.5 m. In the space immediately surrounding the cables, the magnetic field H can reach values of the order of 103 A/m. At ground level, the measurable magnetic induction can reach values of the order of 20-60 xcexcT, depending on the arrangement of the cables with respect to each other.
Although no biological effects due to exposure to magnetic fields of this size generated by low-frequency (50 Hz) sources have yet been demonstrated, there is currently a debate in the scientific community about a possible xe2x80x9csafety thresholdxe2x80x9d, to be adopted by law, below which the probability of biological damage can be reduced to a minimum, if not eliminated. A threshold of magnetic induction on which the scientific community appears to have reached agreement, and on which some national legislation is tending to become harmonized, is 0.2 xcexcT. Thus the value in question is approximately 100 times smaller than that stated previously. The reduction of magnetic induction levels to less than 0.2 xcexcT is certainly to be considered preferable.
The applicant has tackled the problem of screening the magnetic field generated by a high-power electrical power transmission line comprising cables buried in a trench, with the aim of achieving magnetic induction values at ground level of approximately 0.2 xcexcT or less.
The article by P. Argaut, J. Y. Daurelle, F. Protat, K. Savina and C. A. Wallaert, xe2x80x9cShielding technique to reduce magnetic fields from buried cablesxe2x80x9d, A 10.5, JICABLE 1999, describes some solutions for screening magnetic fields generated by three buried cables. In particular, it states the results of some simulations carried out with open-section screens (for example a sheet of ferromagnetic material placed above the cables) and closed-section screens (for example a conduit of rectangular section made of ferromagnetic material, placed around the three cables). The dependency of the screening efficiency on various factors, such as the relative magnetic permeability of the screening material, the thickness of material to be used and the relative positions of the cables and the magnetic screen, is analysed. According to the authors, the optimal material should have a relative magnetic permeability in the range from 700 to 1000 and a thickness in the range from 3 to 5 mm; in the case of a closed-section screen, the optimal relative position is one in which the cables are approximately ⅓ of the way down from the top of the screen. Also according to the authors, attenuation factors of approximately 5-7 can be obtained with open-section screens, factors of approximately 15-20 can be obtained with closed-section screens, factors of approximately 30-50 can be obtained when the closed-section screen is formed very close to the cables (for example from a sheet of ferromagnetic material wound directly around the three cables).
Patent application (Kokai) JP 10-117083 in the name of Nippon Steel Co. provides a solution for the screening of the magnetic field generated by an electrical power transmission cable, consisting of a tube of ferromagnetic material, made by winding a strip of magnetic material in a spiral, preferably on a tubular support, for example a metal or resin pipe. In the example described, the strip is made from grain-oriented steel and has a magnetic permeability which is higher in a direction parallel to the direction of winding than in the direction perpendicular thereto.
The patent EP 606884, also in the name of Nippon Steel, describes a process for producing grain-oriented silicon steel, in which the steel is subjected to a complex rolling process and subsequent stages of annealing, with predetermined times and temperatures, in the presence of recrystallization inhibitors.
The applicant has observed that the solutions described in the article by Argaut et al. cited above do not permit the achievement of very high screening factors, such as those necessary to screen the magnetic field generated by an electrical transmission line.
The applicant has also observed that the solution described in the patent application JP 10-117083 cited above provides for the use of a grain-oriented steel. In this type of steel, the grains have a direction of orientation parallel to the direction of winding: this makes it possible to obtain a very high magnetic flux density. It is produced by complex production processes, which make it possible to orientate the grains only in thin sheets, having thicknesses of the order of a tenth of a millimetre (see, for example, the patent EP 606884 cited above). Because the thickness is so small, the screening tube can be produced only by winding a steel strip in a spiral around a support, as described in patent application JP 10-117083, in order to ensure sufficient mechanical compressive strength. All this makes the process of producing a screening tube extremely complicated.
On the other hand, the applicant has found that it is possible to screen, with an attenuation factor of the order of 100 or above, the magnetic field generated by an electrical power transmission line, by inserting the cables in a conduit comprising at least one layer of non-grain-oriented ferromagnetic material having a high relative magnetic permeability in a range of magnetic field values below 1000 A/m. The screening conduit can advantageously be produced by normal extrusion or rolling methods, without making use of complex production processes for orientating the grains, or by winding as in the aforesaid patent application JP 10-117083.
Here and in the remainder of the description, the term xe2x80x9cnon-grain-oriented materialxe2x80x9d denotes a material in which the crystal domains (grains) essentially have no preferred direction of alignment. The degree of alignment can be evaluated by known methods, for example by optical microscopic analysis, or by X-ray diffractometric analysis. In other words, the material has not been subject to special processes of rolling and annealing, according to the methods used in the production of grain-oriented steel, and the only orientation which may be present in the material is that caused by a normal extrusion or rolling process.
In a first aspect, the invention relates to a method for screening the magnetic field generated by an electrical power transmission line comprising at least one electrical cable, the said method comprising the steps of:
inserting the said cable in a conduit comprising at least one layer of a ferromagnetic material,
characterized in that
the said ferromagnetic material is non-grain-oriented and has a magnetic curve with a maximum value of relative magnetic permeability (xcexcmax) corresponding to a magnetic field value (Hxcexcmax) lower than 1000 A/m.
Preferably, the magnetic curve of the material has a maximum value of relative magnetic permeability (xcexcmax) corresponding to a magnetic field value (Hxcexcmax) in the range from 10 A/m to 800 A/m.
Even more preferably, the magnetic curve has a maximum value of relative magnetic permeability (xcexcmax ) corresponding to a magnetic field value (Hxcexcmax) in the range from 30 A/m to 650 A/m.
Advantageously, the maximum value of relative magnetic permeability (xcexcmax) is at least 500, being preferably in the range from 700 to 5000.
Typically, the method according to the invention comprises the step of burying the conduit in a trench of predetermined depth.
The screening layer can be produced by extrusion, or by the bending of a sheet of predetermined dimensions, for example one produced by rolling, and the subsequent welding of the sheet along its longitudinally opposing sides.
In a preferred embodiment, the method according to the invention additionally comprises the step of arranging the cable or cables in the conduit in such a way that the centre of gravity of a cross section of the cable is in the proximity of the geometrical centre of a corresponding section of the conduit.
Advantageously, the method according to the invention can additionally comprise the step of winding at least one elongated element, for example a cord, around the said at least one cable.
In a second aspect, the invention relates to an electrical power transmission line, comprising:
a conduit comprising at least one layer of ferromagnetic material,
at least one electrical cable inside the said conduit,
characterized in that
the said ferromagnetic material is of the non-grain-oriented type and has a magnetic curve having a maximum value of relative magnetic permeability (xcexcmax) corresponding to a magnetic field value (Hxcexcmax) lower than 1000 A/m.
Preferably, the magnetic curve of the material has a maximum value of relative magnetic permeability (xcexcmax) corresponding to a magnetic field value (Hxcexcmax) in the range from 10 A/m to 800 A/m.
Even more preferably, the magnetic curve has a maximum value of relative magnetic permeability (xcexcmax) corresponding to a magnetic field value (Hxcexcmax) in the range from 30 A/m to 650 A/m.
Advantageously, the maximum value of relative magnetic permeability (xcexcmax) is at least 500, being preferably in the range from 700 to 5000.
In a first example, the ferromagnetic material is a steel whose total content of impurities does not exceed 1.5%.
Preferably, the total content of impurities does not exceed 1%, and even more preferably it does not exceed 0.5%. The said steel with a low content of impurities can be a low-carbon steel: preferably, the carbon content does not exceed 0.16%, and even more preferably it does not exceed 0.03%.
The said steel with a low content of impurities can be a low-manganese steel: preferably, the manganese content does not exceed 1%, and even more preferably it does not exceed 0.5%.
Advantageously, the resistivity of the steel with a low content of impurities is less than 20 xcexcxcexa9xc2x7cm. To improve the screening effect, the grain size index G of the said steel, measured according to the ASTM E-112 standard, is less than 9.
In a second example, the ferromagnetic material is a silicon steel. Preferably, the silicon content is in the range from 1% to 4%.
Advantageously, the electrical power transmission line according to the invention can comprise a support for the cable or cables inside the conduit.
In a preferred embodiment, the electrical power transmission line comprises at least one elongated element wound in a spiral around the said at least one cable, for example a cord made from dielectric material. Preferably, the dielectric material is selected from a group comprising nylon fibres, aramid fibres and polyester fibres.